How do I get the card back for beating Heroic mode in the Curse of Naxxramas?
Solution 1:
You have to complete all 5 wings on heroic mode to unlock the new card back.
Source
Completing Naxxramas on Heroic mode (Quest: The Fall of Naxxramas)
Solution 2:
You don't get it from beating the first wing on heroic.
You have to beat all 5 wings on heroic mode.